1. Field of the Invention
The present invention relates to a photosensor and, more particularly to an image input photosensor.
2. Related Background Art
In a known a conventional image reading apparatus using a one-dimensional line sensor, an original image is focused on the one-dimensional line sensor having a length of several cm by using a reduction optical system, and original image information is then read. In an image reading apparatus of this type, however, a long optical path length is required because the original image is reduced or focused. In addition, the volume of the optical system is large and a compact reading apparatus cannot be obtained.
When a one-to-one magnification optical system having a one-dimensional line sensor having a length equal to the width of the original is used, the volume of the optical system can be greatly reduced, and a compact reading apparatus can be obtained.
FIG. 1 is a schematic view of an image reading apparatus using such an optical system.
Referring to FIG. 1, an original P is fed by a feed roller 41 rotatably supported by a shaft 40 in the f direction. A reader 42 comprises a leaf spring 43 disposed to oppose the feed roller 41 to elastically urge the original P against the feed roller 41 along the entire width of the original P, an LED array 44 serving as an illuminating means for illuminating the original P, an optical system 45 obtained by arranging focusing optical fibers for focusing light reflected by the original P, a transducer 46 including a photoelectric transducer element (photosensor) for photoelectrically converting the focused light, and a holding member 47 for holding the above components.
Upon rotation of the feed roller 41 by a driving means (not shown), the original P urged by the leaf spring or the press member 43 toward the feed roller 41 is guided and fed along the f direction. During feeding, the original surface is illuminated with light from the LED array 44, and light reflected by the original surface is incident on the transducer 46 through the optical system 45. Therefore, the image portions are sequentially read.
The present applicant proposed methods of reading images according to a contact method (Japanese Unexamined Patent Publication Nos. 55-74262, 55-75271, 56-45084, and 56-122172) without using such optical fibers and such a lens array. According to the contact method, an original is moved in contact with a one-dimensional line sensor to read the image from the original.
FIG. 2 shows such an image reading apparatus. An incident light window 51 is formed in a mounting base 50, and light from a light source 52 is incident through the incident window 51. A transducer 53 and an electronic circuit 54 of an IC for processing an optical signal from the transducer 53 are arranged on the mounting base 50. Light emitted through the incident light window 51 is incident through the transducer 53 on the original P fed by a feed roller 55. Light reflected by the original P is read by the photosensor in the transducer 53.
Typical photosensors used in such an image reading apparatus are a thin-film sensor obtained by using a photoconductive film and an amorphous silicon film (a-Si), and a multi-chip sensor using a plurality of CCD sensor chips.
The a-Si film element has a function for storing optical carriers in a capacitor in an element. A light-sensitive time is the same as one-line scanning time. Therefore, the a-Si film element is excellent in sensitivity, optical response which are better than those of CdS and CdSe. However, as compared with single-crystal Si, a-Si has a lower mobility and is not suitable for high-speed reading.
Although a CCD sensor consisting of single-crystal Si can be operated at higher speed than that of the a-Si film sensor and higher sensitivity than that, a single-crystal Si sensor is arranged by connecting a plurality of chips Therefore, a high packing density cannot be obtained, and signal processing is complicated, thus increasing cost
FIG. 3 is a schematic view of an image reading apparatus using a one-to-one magnification optical system.
Referring to FIG. 3, an original P is fed by a feed roller (not shown) in the f direction, and a sensor unit 150 arranged under the lower surface of the original P reads original image information. The unit 150 includes a light source 151 serving as an illuminating means for illuminating the original P along the entire width of the original P, an optical system 152 of a SELFOC lens for focusing light reflected by the original P at a one-to-one magnification, and a photoelectric transducer 153 for photoelectrically converting light reflected by the original P.
The transducer 153 causes a photosensor to photoelectrically convert light into electrical signals and sequentially sending the electrical signals to a signal processing unit in response to a clock signal from a control clock generator 154.
In the signal processing unit 155, an analog signal photoelectrically converted by the transducer 153 is converted into a digital signal by an A/D converter 156, and variations in sensitivity of the converted signals are corrected by a correction table memory 157 and a correction data memory 158. Therefore, a corrected signal is output from the signal processing unit 155. The above components in the signal processing unit 155 are controlled by the clock signal from a control clock generator 159.
FIGS. 4 and 5 are a perspective view and an equivalent circuit diagram, respectively, of the photoelectric transducer 153.
Referring to FIGS. 4 and 5, a photosensor unit 161 comprising photosensors S1 to Sn , a storage capacitor unit 162 comprising capacitors C1 to Cn, and a transfer unit 163 comprising transfer switches T1 to Tn and flip-flops F1 to Fn are formed on a substrate 160. The flip-flops F1 to Fn constitute a shift register. A bias voltage Vs is applied to the photosensors S1 to Sn, and clock pulses CP are supplied from a control clock generator 154 to clock terminals of the flip-flops F1 to Fn.
With the above arrangement, original image information is converted into a photocurrent by the photosensors S1 to Sn. The photocurrent is stored as a storage charge in the storage capacitors C1 to Cn for a predetermined period of time. The stored charge is output through the transfer unit 163.
When a normal silicon chip is used for a drive IC chip constituting the transfer unit in the conventional reading apparatus, the number of bonding portions between the photosensors and the IC chip is very large, resulting in high cost and decreasing a product yield.
For example, when an A4-size original is read at a resolution of 400 pixels/inch, the number of photosensor elements is about 3,500. For this reason, when the number of bits per IC chip is given as 64 bits, 54 IC chips are required, and the number of bonding wires is as large as 3,500 to 5,000.
For the above reason, a conventional transfer unit comprises thin film transistors consisting of polycrystalline or amorphous silicon and formed on a substrate on which photosensors are formed. However, the electron mobilities of the polycrystalline and amorphous silicon are as small as 1/100 to 1/1000 that of single-crystal silicon. The read speed of such an arrangement is limited For example, it takes about 15 to 30 seconds to read each A4-size original. When this photosensor arrangement is used in a facsimile machine, only a low-speed machine is prepared.
Variations in photosensor sensitivity depend on individual photoelectric transducer units, and correction data must be prestored in a correction data memory prior to the read operation. As the correction data memory comprises a read/write memory (RWM) the contents of the memory must be backed up by a battery or the like after the power switch is turned off.
It is possible to store correction data in a read-only memory (ROM) as the correction data memory. In this case, variations in individual photosensor sensitivity in each photoelectric transducer unit must be measured, and the measurement results must be written in the ROM. This ROM must be provided to each transducer unit. Therefore, assembly, parts control, and transport control are complicated. For example, combinations of transducer units and ROMs may be mistaken, thus degrading reliability of the apparatus.
Different control clocks are required for an operation for reading the charge from the transducer unit and an operation for reading correction data from the signal processing unit due to the following reason. The transducer unit must be driven by an n-stage shift register controlled by shift pulses, and a special power source system which satisfies the characteristics of the photosensors and the transfer switch must be driven. However, the signal processing unit is driven such that the memory is properly addressed. Therefore, two control clock generators must be used, thus increasing the cost.
A compact, low-cost image reading apparatus suitable for a high-performance line sensor cannot be proposed in which production control and steps can be simplified.